This invention relates to methods, compositions, and various techniques for improving, i.e., increasing, the temperature at which various materials, particularly metal oxide composites, will exhibit superconductivity.
At ordinary temperatures, the electrical conductivity of substances extends over a tremendous range, from about 1.6.times.10.sup.-8 ohm.cm for silver to at least 10.sup.16 ohm.cm for such dielectrics as quartz. The range is even greater at very low temperatures.
Superconductivity is a property of materials characterized by essentially zero electrical resistivity (i.e., infinite conductivity). Superconductivity was believed to occur in elements having two to five valence electrons outside the closed shell. It occurs at a transition temperature below which the material is superconducting and above which the material is not.
Superconductivity was discovered in 1911, just three years after refrigeration techniques were invented that could liquefy helium at 4 degrees Kelvin (i.e., "4K"). A Dutch scientist, H. Kammerlingh Onnes, found that mercury lost all resistance to electricity when cooled in the liquid helium. Superconductivity was thereafter confirmed in other metals when cooled with liquid helium. In the early 1970's, scientists discovered metal alloys of niobium and germanium that exhibited superconductivity at temperatures as high as 23K. A search for still higher temperature superconductors was largely unsuccessful at that time.
Superconductivity is conventionally explained on the hypothesis that virtual distortions in the metal ion lattice provide an attractive interaction between conduction electrons which causes them to form bound pairs. At sufficiently low temperature, these pairs "condense" into a superconducting state separated by an energy gap from the normal conducting state. In the superconducting state, collisions with lattice vibrations or impurities do not have enough energy to excite the system above the energy gap, and supercurrents flow without resistance. In the normal conducting state there is no energy gap, and collisions of the electrons with lattice vibrations and impurities excite the system, giving rise to energy losses. These energy losses appear as a finite resistivity.
In a superconductor, according to the conventional theory, electrons see an attractive interaction arising from distortions of the metal ion lattice. As the first electron moves through the lattice, its negative charge repels the negative electron cloud and attracts the positively charged ions, causing the lattice structure to appear to pucker or squeeze together. The second electron is attracted to this concentration of positive charge. The second electron "follows" the first, making it seem as though the two are attracted to each other.
In 1986, the scientific world began reporting the achievement of superconductivity at higher temperatures. However, the concepts used to explain superconductivity in the older materials do not work well for the new superconductive materials. As the superconducting transition temperature of a material increases, for example, the old formulae suggests that the electron pairs must be bound together more strongly to keep from being dissociated by thermal energy. According to the old formulae, the binding force should not be strong enough to hold the electron pairs together at the higher temperatures.
Many theories have been proposed to explain the electron pairing. However, none of them have provided a basis for improving the superconductive properties of a material. Similarly, the new theories have not provided an adequate basis for explaining many of the observed phenomena of superconductivity.